1. Field
This invention relates to ion-sensitive electrodes for measuring ionic activity or concentration and, more particularly, to an electrode having substantial strength and ruggedness to facilitate use directly in industrial processes.
2. Description of the Prior Art
Measurements of ionic activity (or concentration) have long been made by inserting into a test liquid a pair of electrodes at least one of which may be provided with a fragile membrane selectively responsive to the particular ion of interest. One common type of such measurement is that of hydrogen ion activity (pH), conventionally made by a pair of electrodes referred to respectively as a measuring glass electrode and a reference electrode. Such electrodes develop an electrical potential (emf) which is a function of hydrogen ion activity. This potential is detected by an amplifier which produces an output usable for driving an indicator or a controller device.
The glass electrode generally comprises a cylindrical tubular element of glass closed off at one end by a thin bulbous glass membrane formed of a special composition sensitive to hydrogen (or other) ions. The outer surface of this membrane is immersed in the test liquid and makes electrical contact therewith. Being within the tubular element and isolated from the test liquid by the membrane, an electrically-conductive buffered liquid (electrolyte) is used for wetting the inner surface of the glass membrane so that the electrical potential responsive to the ion activity of the test liquid can be developed across the membrane.
Immersed in the electrolyte is a circuit-completing internal element formed for example of silver/silver-chloride and connected to an output extending to one input terminal of an amplifier. The other amplifier input terminal is connected to the reference electrode. These two electrodes form, together with the test liquid, an electro-chemical cell which generates an electrical potential proportional to the hydrogen-ion activity. The individual glass or reference electrodes are at times referred to as "half cells."
Although conventional glass electrodes have been used successfully for many years in making laboratory measurements and the like, there have been problems in employing such electrodes widely in industrial process fluid flows containing hard solid particles. Because it is so thin, the glass electrode membrane is relatively delicate and thus its ability to withstand mechanical shock and other abuse is limited. Moreover, there are processes (such as food) where the use of these electrodes is generally obviated because broken glass fragments cannot be tolerated in the fluid streams of those processes. Accordingly, there is a need for a ruggedized glass electrode. But it should be explained that it is not possible simply to make the glass membrane more rugged merely by increasing its thickness, for a thick membrane would present too high an electrical resistance and measurement of the electrical potentials developed would not be possible with commercially available high input-impedance voltage-measuring equipment.
U.S. Pat. No. 3,717,565, issued on Feb. 20, 1973, to Doyle, and assigned to the present assignee, discloses a ruggedized measurement glass electrode which includes a homogeneous integral mass of ion-sensitive material that is joined to and forms the lower portion of a tubular container holding an electrolyte. The portion of the mass disposed within the container is a disc-shaped honeycomb member having holes extending downwards from openings formed on the top surface of the mass and terminating in blind ends that are positioned a short distance from the bottom surface of the mass. The top surface is arranged within the container to be exposed to the electrolyte and the bottom surface, being the outer bottom surface of the container, is exposed to a test solution or fluid stream having an unknown ion activity. The thickness of the regions between the ends of the holes and the bottom surface is sufficiently small so as to be equivalent to the thin ion-sensitive membrane of the previously described prior art electrode. Since the ion-sensitive material of the integral mass is non-porous, the holes allow the electrolyte to contact the blind ends thereof so that a measurable electrical potential across the regions can be generated responsive to the ion activity of the solution contacting the second surface of the mass.
However, fabrication of the glass electrode disclosed by the '565 patent requires painstaking drilling, molding and/or machining techniques along with expensive specialized equipment in order to produce sufficiently thin regions between the blind ends and the bottom surface of the integral mass. As a result of the above requirements, the costs of producing the electrode taught by the Doyle patent are prohibitive.
In U.S. Pat. No. 3,855,098, issued on Dec. 17, 1974, to the present inventor and assigned to the present assignee, there is disclosed another ruggedized electrode in which the integral mass taught by Doyle is replaced by an assembly made by fusing a thin ion-sensitive glass membrane to a surface of a mechanically rigid back-up member. The back-up member is made of an inert (i.e., not ion-sensitive) porous ceramic having a coefficient of thermal expansion substantially matching that for the membrane. This arrangement greatly reduces the production costs because conventional and less expensive fusing techniques can be used to join the thin membrane to the back-up member. Being porous, the back-up member permits the electrolyte to be transported therethrough for wetting the interior surface of the membrane so that measurable electrical potentials can be generated across the membrane.
The making of the electrodes taught by the '098 patent is relatively straightforward for electrodes with glass membranes having low thermal expansion characteristics (such as those for sodium ion-sensitive formulations). However, for electrodes made with glass membranes having large thermal expansion characteristics (such as those for hydrogen ion-sensitive glasses), problems have been encountered in fabricating electrodes that perform comparably and/or are as durable or reliable as the above-mentioned sodium ion-sensitive electrodes. In regard to hydrogen ion-sensitive electrodes, the thermal expansion characteristics of available ceramics no longer match those for hydrogen-sensitive membranes within the 2% figure mentioned in the '098 patent, so that the glazing procedure used to produce the comparable electrodes becomes very complex and difficult.
As a result of the above, there is still a need for pH glass electrodes which are ruggedized to withstand mechanical shock and physical abuse and are usable in industrial process streams for measuring the ionic activity or concentration of a wide range of prescribed ions.